In the production of a silicon single crystal, a method called the Czochralski process (the CZ process) is widely adopted. In the production of a silicon single crystal by the Czochralski process, in general, the inside of a quartz glass crucible (also called a quartz crucible) is filled with polycrystalline silicon (polysilicon), the polycrystalline silicon (polysilicon) is melted by heating to obtain silicon melt, a seed crystal is immersed in the silicon melt and then pulled upwardly, whereby a silicon single crystal ingot is grown.
In the past, it has been pointed out that, during the growth of a silicon single crystal, an air bubble contained in a quartz glass crucible expands under high temperature and an inner periphery of the crucible is delaminated, resulting in the generation of dislocation in the silicon single crystal (refer to Patent Document 1, for example) and the surface of the quartz glass crucible changes from an amorphous state to cristobalite and the cristobalite is delaminated, resulting in the generation of dislocation in the silicon single crystal (refer to Patent Document 2, for example).
As for cristobalitization (crystallization) of the surface of the quartz glass crucible during the production of a silicon single crystal by the Czochralski process, according to Patent Document 3 and Patent Document 4, there are statements “crystallization occurs in the form of dots from a nucleus for crystal formation as a starting point at an early stage of crystallization, and the crystallization spreads in the form of a ring with the progress of pulling of a single crystal”, “with such progression of the crystallization, a crystallization spot is generated. Since the outer periphery of this crystallization spot takes on a brown color, the crystallization spot is sometimes called a brown spot.”, and “the crystallization spot increases with the passage of the single crystal pulling time, that is, the time in which the silicon melt makes direct contact with the inner surface of the quartz crucible, but, after a lapse of a predetermined time, the density of a crystallization spot converges to a constant density and remains at that density”. Moreover, there is also a statement “after such a crystallization spot is generated, the crystallization spot begins to melt by the silicon melt and the size of the crystallization spot becomes gradually small”.
The cristobalitization of the surface of the quartz glass crucible is said to be promoted when the crucible contains a high concentration of impurities such as alkali metal. Moreover, in terms of the influence on device characteristics, a low impurity concentration is preferable. Therefore, the quartz glass crucible is required to contain no air bubble and contain a low concentration of impurities.
Examples of a method for producing synthetic quartz glass containing no air bubble and containing an extremely low concentration of impurities include a direct process and a soot process. The direct process is a synthetic method to directly deposit and vitrify synthetic quartz glass by hydrolyzing a silicon compound such as silicon tetrachloride (SiCl4) in oxyhydrogen flames. Moreover, the soot process is a method for producing synthetic quartz glass by the following procedure. First, by hydrolyzing a silicon compound such as silicon tetrachloride (SiCl4) in oxyhydrogen flames at a temperature of about 1100° C. which is lower than the temperature of the direct process, a mass (soot) of porous silica is synthesized. This soot is treated with heat in an appropriate gas such as a chlorine compound to remove water therefrom. Then, the soot is heated from the lower end thereof and is vitrified by being pulled downwardly while being rotated at a temperature of about 1500° C. or higher (refer to Non patent Literature 1).
By making a quartz glass crucible by using such synthetic quartz glass, it is possible to avoid generation of dislocation in a silicon single crystal, but the heat resistance (also called resistance to heat deformation and deformation resistance) of the crucible itself is low (in other words, the crucible tends to be deformed under high temperature). As a method for solving this problem related to heat resistance, for example, there are (1) a method by which synthetic quartz glass synthesized from a silane compound is pulverized, heated and melted under vacuum, and molded into a crucible (Patent Document 5) and (2) a method by which a synthetic quartz glass material produced by a direct flame method performed on a silane compound, the synthetic quartz glass material whose hydrogen molecule content is 1×1017 molecules/cm3 or more, is turned into synthetic quartz glass powder after undergoing pulverization, particle size adjustment, and cleaning processes, and the synthetic quartz glass powder is electrically melted under vacuum at 1500 to 1900° C. and is then molded (Patent Document 6).
With the method of Patent Document 5, by pulverizing synthetic quartz glass by setting the particle size at the time of pulverization at 600 μm or less and performing vacuum pressure melting thereon at a pressure of 10−1 Torr at 1500 to 1900° C. to reduce the content of hydroxyl group and chlorine, it is possible to make a synthetic quartz glass crucible with high heat resistance. As a result of the vacuum pressure melting having being performed, an air bubble having a size of 1 mm or more is not contained in the crucible. This is better than the air bubble level (for example, about three air bubbles having a size of 1 to 2 mm and no air bubble having a size of 2 mm or more per crucible) of a quartz glass crucible produced by a normal arc melting method. The arc melting method is a method for producing a quartz glass crucible by feeding raw material powder into a rotating mold to form a raw material powder layer in the crucible shape and heating the raw material powder layer by arc discharge from the inside thereof and melting it (refer to Patent Document 7, for example).
Moreover, according to the method of Patent Document 6, a synthetic quartz glass material which is made to have a hydrogen molecule content of 1×1016 molecules/cm3 or more, a strain point of 1130° C. or higher, and an OH group content of 1 ppm or less and a chlorine content of 1 ppm or less is highly-pure and is allowed to have a viscosity at high temperature of 1010 poise or more at 1400° C., for example, making it possible to use this as a material for a crucible for pulling a silicon single crystal.
Furthermore, Patent Document 8 discloses a method by which quartz glass pieces obtained by melting quartz raw material powder in an atmosphere of inert gas and purifying it by keeping it at 2000° C. or higher at 0.05 torr or more of vacuum for 5 hours or longer are bonded to the inner surface of a quartz glass crucible and merging them together by heating and melting. Moreover, as the heating and melting method, the use of arc discharge, an oxyhydrogen flame burner, and the like is illustrated as an example.